Ligand effect on the catalytic activity of porphyrin-protected gold ...

Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2017

Supporting Information

Ligand effect on the catalytic activity of porphyrin-protected gold clusters in the electrochemical hydrogen evolution reaction Daichi Eguchi,a Masanori Sakamoto,b* Toshiharu Teranishib* a

Department of Chemistry, Graduate School of Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011,

Japan b

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

1. Chemicals All

reagents

were

used

as

received

without

further

purification.

Tetrakis-5α,10α,15α,20α-(2-

acetylthiomethylphenyl)porphyrin (SC1P) was synthesized from terephthalaldehydic acid in five steps.1 Tetrakis5α,10α,15α,20α-(2-acetylthioethylphenyl)porphyrin (SC2P) was synthesized from 3,4-dihydro-1H-2-benzopyran in three steps.1

2. Characterization Low temperatures (–80 or –50 C) were maintained using a liquid nitrogen–methanol cooling bath. Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) was performed on a Bruker Autoflex Speed system using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as a matrix. The cluster-to-matrix molar ratio was set to 1:100. Gel permeation chromatography (GPC) was carried out on a BIO RAD sx-1 system, using toluene or N,N-dimethylformamide (DMF) as the mobile phase. Gel permeation chromatography–high performance liquid chromatography (GPC–HPLC) was performed on a LC-9225 NEXT system (Japan Analytical Industry Co., Ltd.) using a JAIGEL-W253 column. A DMF solution of lithium bromide (50 mM) was used as the mobile phase at a flow rate of 3.8 mL min–1. Chromatograms were collected by monitoring at a wavelength of 435 nm. Inductively coupled plasma–atomic emission spectroscopy (ICP–AES) was performed on a Shimadzu ICPE-9800 system. UV-vis absorption spectra were measured at room temperature on a Hitachi U-4100 spectrophotometer. Fluorescence spectra were recorded at room temperature on a Horiba Fluorolog-3. Transmission electron microscopy (TEM) was performed using a JEOL JEM-100 microscope operated at an acceleration voltage of 100 kV. Histograms of the core diameters for the AuCs and AuNPs were constructed by measuring the sizes of 500 particles. X-ray photoemission spectroscopy (XPS) was performed using a PHI Quantum 2000 spectrometer with an Al Kα radiation source. XPS spectra were calibrated against the C 1s peak at 284.8 eV.

3. Electrochemical measurements Electrochemical measurements were performed on an ALS611 electrochemical workstation with a standard three electrode cell. An Ag/AgCl electrode and a platinum wire were used as the reference electrode and counter electrode, respectively. Working electrodes were prepared as follows: a DMF or toluene solution of ligand-protected AuCs and AuNPs (1 mM of Au atoms, 30 μL) was dropped onto carbon tape (Toray, TGP-H-090, 1 × 3 cm2). Carbon tape containing sample was dried in vacuo overnight. Linear sweep voltammetry (LSV), beginning at 0.1 V and ending at –0.8 V vs. reversible hydrogen electrode (RHE) with a scan rate of 100 mV s–1, was conducted in potassium phosphate buffer solution (0.5 M, pH 6.7). Argon gas was bubbled through the electrolyte vigorously for 15 min to remove oxygen in the solution. The measured potentials vs. Ag/AgCl were converted to the RHE. The iR-drops by the solution were also corrected using the following equation: E (vs. RHE) = E (vs. Ag/AgCl) + 0.242 + 0.059 × pH – iRΩ The error bars indicate the standard deviations from three runs using freshly deposited samples. All LSV curves in this work were obtained with the first scan.

4. Experimental Section Synthesis of SCnP-protected AuCs (SCnP/AuCs) An aqueous solution of hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4∙4H2O; 5 mL, 29.9 μmol) was added to a CH2Cl2 solution of tetraoctylammonium bromide (TOABr; 5 mL, 34.7 μmol), and the mixture was stirred vigorously for 10 min to transfer Au(III) ions into the organic phase. After removal of the aqueous phase, a mixture of SCnP (14.7 μmol) in CH2Cl2 (25 mL) and methanol (30 mL) was added to the CH2Cl2 solution of Au(III). The mixture was stirred and cooled at –80 C for 30 min. A solution of sodium borohydride (NaBH4, 305 μmol) in methanol (1 mL) was added to the CH2Cl2 solution of Au(III) and SCnP, and stirred at room temperature for 3 h to obtain AuCs. After removal of the solvent in vacuo, the residue was washed with methanol and collected by filtration. The crude product was purified by GPC–HPLC to obtain the SCnP/AuCs. Chemical compositions of the obtained SCnP/AuCs were different from those in our previous work.1 The Au(III) reduction temperature was slightly higher than in previous work. Therefore, the growing rate of these cores slightly became faster.

Synthesis of SCnP-protected 2.2-nm AuNPs (2.2-nm SCnP/AuNPs) Solutions of HAuCl4∙4H2O (14.3 μmol) in methanol (7.5 mL) and SCnP (2.79 μmol) in DMF (7.5 mL) were combined and stirred at –50 C for 30 min. A solution of NaBH4 (161 μmol) in methanol (1 mL) was then added to the mixture of Au(III) and SCnP, and stirred at –50 C for 30 min and room temperature for 30 min to obtain AuNPs. The reaction solution was then centrifuged with methanol (75 mL) and the resultant precipitate was redispersed in DMF and purified by GPC–HPLC to obtain 2.2-nm SCnP/AuNPs.

Synthesis of SCnP-protected 3.8-nm AuNPs (3.8-nm SCnP/AuNPs) A solution of HAuCl4∙4H2O (28.2 μmol) and SCnP (5.37 μmol) in DMF (15mL) was stirred at 0 C for 30 min, before adding a solution of NaBH4 (267 μmol) in methanol (1 mL). This solution was stirred at 0 C for 30 min and room temperature for 30 min to obtain AuNPs. The reaction solution was centrifuged with methanol (75 mL) and the resultant precipitate was redispersed in DMF and purified by GPC to obtain 3.8-nm SCnP/AuNPs.

Synthesis of phenylethanethiol (PET)-protected AuCs (PET/AuCs) An aqueous solution of HAuCl4∙4H2O (2.5 mL, 39.6 mg, 96.0 μmol) was added to a solution of TOABr (63.3 mg, 116 μmol) in CH2Cl2 (20 mL) and stirred vigorously for 10 min to transfer Au(III) ions to the organic phase. After removal of the aqueous phase, a solution of phenylethanethiol (PET, 49.7 μL, 390 μmol) in CH 2Cl2 (128 mL) and methanol (30 mL) was added. The mixture was stirred at room temperature for 90 min and cooled at –80 C for 30 min. A solution of NaBH4 (35.4 mg, 930 μmol) in methanol (2 mL) was added, and the solution was stirred at room temperature for 1 h to obtain AuCs. After removal of the solvent in vacuo, the residue was washed with methanol and collected by filtration. The crude product was purified by GPC–HPLC to obtain PET/AuCs.

Synthesis of PET-protected 2.3-nm AuNPs (2.3-nm PET/AuNPs) A solution of chlorotriphenylphosphine gold(I) (50.1 mg, 101 μmol) and PET (26.8 μL, 200 μmol) in CHCl3 (10 mL) was stirred at 65 C for 15 min. A solution of tert-butylamine borane complex (86.5 mg, 1.01 mmol) in CHCl3 (1 mL) was then added and the resultant mixture was stirred at 65 C for 9 h. After removal of the solvent in vacuo, the residue was washed with methanol and collected by filtration. Finally, the resultant precipitate was redispersed in toluene.

Synthesis of PET-protected 3.8-nm AuNPs (3.8-nm PET/AuCs) This synthetic procedure was slightly modified from previous work.2 Citrate-protected 3.8-nm AuNPs (CT/AuNPs) were prepared according to the literature.3 An aqueous solution (24 μmol of Au atoms, 100 mL) of CT/AuNPs was added to a mixed solution of TOABr (54.4 mg, 99.5 μmol) and PET (134 μL, 1.00 mmol) in toluene and acetone (35 mL, 6:1 v/v) and stirred at room temperature for 30 min. After removal of the aqueous solution by syringe, the organic phase was evaporated in vacuo and the residue was washed with methanol. Finally, the residue was redispersed in toluene.

Synthesis of PET-protected 7.3-nm and 14.8-nm AuNPs (7.3-nm PET/AuNPs and 14.8-nm PET/AuNPs) These synthetic procedures were slightly modified from previous work.4 The 7.3-nm and 14.8-nm CT/AuNPs were prepared according to the literature.3 An aqueous solution of CT/AuNPs (12 μmol of Au atoms, 50 mL) was added to a mixture of toluene and oleylamine (22 mL, 10:1 v/v), and stirred at 100 C for 1 h. After removal of the aqueous solution by syringe, the organic phase was evaporated and the resulting mixture was centrifuged with ethanol (80 mL). Finally, the resultant precipitate was redispersed in toluene to obtain oleylamine-protected AuNPs (7.3-nm OAm/AuNPs and 14.8-nm OAm/AuNPs). A solution of OAm/AuNPs (1 mM of Au atoms) in toluene (30 μL) was dropped on carbon tape (1 × 3 cm2) and dried in vacuo overnight. Carbon tape with AuNPs was dipped into a solution of PET (436 μL, 3.0 μmol) in ethanol (20 mL) at room temperature for 3 h. The carbon tape was then washed with ethanol and used as a working electrode.

Figure S1. GPC–HPLC chromatograms of crude products SC1P/AuCs (red) and SC2P/AuCs (blue). Fractions were collected for further characterization.

Figure S2. TEM images (left) and corresponding histograms (right) representing the size distributions of (a) 1.3 nmSC1P/AuCs, (b) 1.3-nm SC2P/AuCs, and (c) 1.2-nm PET/AuCs.

Figure S3. MALDI–TOF MS spectra of (a) SC1P/AuCs and (b) SC2P/AuCs in linear positive mode.

Figure S4. (a) Schematic illustration of SC2P coordination with AuCs. (b) Structure of SC2P from top view.1

Table S1. Number of SCnP ligands coordinated on AuCs and AuNPs. Compounds

Radius (x in Figure S4, nm)

Distance between sulfur and porphyrin ring (y in Figure S4, nm)[a]

Coordination number of SCnP on Au surface (Estimation)[b]

Coordination number of SCnP on Au surface (Experimental value)

SC1P/AuCs

0.65

0.34

10

8[c]

2.2 nm SC1P/AuNPs

1.1

0.34

21

25[d]

3.8 nm SC1P/AuNPs

1.9

0.34

52

57[d]

SC2P/AuCs

0.65

0.49

13

11[c]

2.2 nm SC2P/AuNPs

1.1

0.49

26

36[d]

3.8 nm SC2P/AuNPs

1.9

0.49

59

71[d]

[a]Estimation [b]Values

from single X-ray crystal structures.1

were estimated using the following equation:

(4 × π × (Radius (nm) + Distance between sulfur and porphyrin ring (nm))2)/(occupied area of porphyrin derivative (1.2 nm2)) [c]Estimated [d]Values

from ICP–AES and MALDI–TOF MS.

were estimated from ICP–AES and calculated average number of Au atoms in NP. Au:S molar ratio was investigated using

ICP–AES measurements and used to determine the Au:SCnP molar ratio given that SCnP contains four sulfur atoms in acetylthio groups. Average number of Au atoms in NP was calculated according to previous literature. 5 Number of SCnP on the Au surface was estimated from the ratio and calculated average number of Au atoms in NP. For example, for 2.2-nm SC1P/AuNPs, the calculated average number of Au atoms was 329, and the Au:SC1P molar ratio was 329:25, giving the number of coordinated SC1P on the Au surface as 25.

Figure S5. TEM images (left) and corresponding histograms (right) representing the size distributions of (a) 2.2-nm SC1P/AuNPs, (b) 2.2-nm SC2P/AuNPs, and (c) 2.3-nm PET/AuNPs.

Figure S6. TEM images (left) and corresponding histograms (right) representing the size distribution of (a) 3.8-nm SC1P/AuNPs, (b) 3.8-nm SC2P/AuNPs, and (c) 3.8-nm PET/AuNPs.

Figure S7. TEM images (left) and corresponding histograms (right) representing the size distributions of (a) 7.3-nm OAm/AuNPs, and (b) 14.8-nm OAm/AuNPs.

Figure S8. Absorption spectra of free SCnP with (a,b) 2.2-nm SCnP/AuNPs and (c,d) 3.8-nm SCnP/AuNPs in DMF solution. Molar concentrations of SCnP and SCnP on AuNPs were 2.0 μM.

Current density (mA cm-2)

0 -2 PET/AuCs 2.2 nm PET/AuNPs 3.8 nm PET/AuNPs 7.3 nm PET/AuNPs 14.8 nm PET/AuNPs

-4 -6 -8

-0.45 -0.40 -0.35 -0.30 -0.25 Potential (V vs. RHE)

Figure S9. Comparison of current densities for PET-protected AuCs and AuNPs of various sizes at different

0.48

(V vs. RHE)

Overpotential at 10 mA cm-2

potentials. Error bars indicate standard deviations from three runs with freshly deposited samples.

0.50 0.52 0.54 0

2

4

6 8 10 12 14 16 Size (nm)

Figure S10. Overpotentials for PET-protected AuCs and AuNPs of various sizes at 10 mA cm–2. Error bars indicate standard deviations from three runs with freshly deposited samples.

Estimation of the PET coverage on AuC for comparison with that of SCnP/AuCs The surface area of AuCs (d = 1.2 nm) was calculated as follows, 4 × π × (0.6 nm)2 = 4.5 nm2

(1)

Area per PET molecule on a AuC surface was estimated to be 0.16 nm2 ligand-1 according to the previous literature.6 The length of PET was estimated by Avogadro 1.1.1 program. The MALDI-TOF MS and ICP-AES indicated that the chemical composition of PET/AuCs was Au39(PET)26. Therefore, Surface coverage of PET was, 26 × 0.16 nm2 = 4.2 nm2

(2)

Using equation (1) and (2), the PET coverage was, 4.2 nm2/ 4.5 nm2 = 0.93

Figure S11. Absorption spectra of (a) SC1P/AuCs and (b) SC2P/AuCs after HER in DMF.

(3)

Figure S12. Plot of fluorescence intensity versus (a) molar concentration of 2-naphthalenethiol (2-NT), and the mixed solution of 2-NT and (b) SC1P/AuCs (1.6 μM) and (c) SC2P/AuCs (1.6 μM) excited at 283 nm in CH2Cl2 and DMF (1:1 (v/v)). The molecular accessibility was carried out based on previous work.7 First, the 2-NT attached on the surface of AuCs resulted in quenching the fluorescence (red region). After saturation of adsorption sites on the Au surface, the free 2-NT showed fluorescence (blue region).

Figure S13. XPS spectra of SC1P/AuCs (red) and SC2P/AuCs (blue) in (a) Au4f and (b) Au5d. Binding energy differences between SC1P/AuCs and SC2P/AuCs of Au4f7/2 and Au5d5/2 were 0.28 and 0.44 eV, respectively. Notably, these differences were not derived from their charging up.

Figure S14. HER polarization curves for ligand-protected (a) 2.2-nm and (b) 3.8-nm AuNPs in 0.5 M phosphate buffer solution (pH 6.7). Comparison of current densities for ligand-protected (c) 2.2-nm and (d) 3.8-nm AuNPs at different potentials. Error bars indicate standard deviations from three runs with freshly deposited samples.

Table S2. Optical properties of SCnP, SCnP/AuCs, and SCnP/AuNPs. Compounds

λmax (nm)

ε (×105 M-1 cm-1)

FWHM (nm)

SC1P

420

3.9

13

SC1P/AuCs

425

1.1

21

2.2 nm SC1P/AuNPs

426

0.48

27

3.8 nm SC1P/AuNPs

426

0.26

34

SC2P

418

3.9

12

SC2P/AuCs

422

2.1

19

2.2 nm SC2P/AuNPs

422

0.65

19

3.8 nm SC2P/AuNPs

424

0.56

18

Table S3. Summary of catalytic parameters for SCnP/AuCs, SCnP/AuNPs, PET/AuCs, and PET/AuNPs in the HER. Compounds

Onset potential (mV)

Overpotential at 10 mA cm−2 (mV)

Current density at −0.4 V vs RHE (mA)

Tafel slope (mV dec−1)

SC1P/AuCs

−390

470

−2.8

106

SC2P/AuCs

−440

500

−0.84

88.6

2.2 nm SC1P/AuNPs

−330

460

−4.8

101

2.2 nm SC2P/AuNPs

−380

470

−3.4

105

3.8 nm SC1P/AuNPs

−380

470

−3.7

114

3.8 nm SC2P/AuNPs

−390

490

−2.5

111

PET/AuCs

−440

540

−0.61

101

2.3 nm PET/AuNPs

−370

490

−3.6

87.8

3.8 nm PET/AuNPs

−400

500

−3.5

96.6

7.3 nm PET/AuNPs

−420

510

−2.4

82.3

14.3 nm PET/AuNPs

−470

530

−1.7

91.3

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